My suggestion is that it is a larger capacity magazine that has had it's capacity limited to either 15 rounds (the old legal limit) or 10 rounds (complying with current NJ limits). I would suspect that this magazine was owned by someone that lives in a free state and shot often in NJ. The way the homemade label looks, it may been how the owner segregated their magazines from free state versus NJ legal.

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Judge control not gun control!